Resolving OAS Validation Errors for Duplicate Paths

Key Takeaways

The Root Cause: An OpenAPI (OAS) "duplicate path" error occurs because paths like /users/{id} and /users/{name} are considered structurally identical. The validation checks the path template, not the parameter names.
Why It's Forbidden: This rule exists to prevent non-deterministic routing. An API gateway or server cannot reliably know whether a request for /users/jsmith refers to an ID or a username, leading to unpredictable behavior.
The Solutions:
1. Unique Static Segments (Recommended): The clearest fix is to make paths structurally different, e.g., /users/id/{id} and /users/name/{name}.
2. Query Parameters: Use a single collection endpoint (/users) and filter with query parameters, like /users?id=123 or /users?username=jsmith.
The Gateway's Role: An API gateway is central to this issue. It enforces routing based on the spec and can serve as an abstraction layer to expose a clean, unambiguous API to the public, even if it's routing to messy, conflicting legacy services internally

The Router's Dilemma: What is an Ambiguous Path?

You've carefully crafted your OpenAPI specification, defining endpoints to fetch users by their unique ID and by their username. You feel confident in your design. Then, you run a linter or try to import the spec into an API gateway, and it screams an error: Path already exists, no-identical-paths, or Ambiguous path. You stare at the two paths:

/users/{userId}
/users/{username}

They look different! One has userId, the other has username. What's going on?

The issue lies in how the OpenAPI Specification and the tools that implement it interpret path templates. According to the specification, the structure of the path is what determines its identity, not the names of the parameters within the curly braces. Paths like /pets/{petId} and /pets/{name} are considered functionally identical and therefore invalid. Both are equivalent to the template /pets/{something}.

The core problem is non-deterministic routing. Let's personify the API router or gateway for a moment. When it receives an incoming request for GET /users/jsmith, it faces a dilemma. How is it supposed to know whether jsmith is a userId or a username without inspecting the value's content? Should it try to parse it as an integer? What if your usernames can also be numeric? This is an ambiguity that forces the router to guess, which leads to unpredictable behavior, bugs, and security vulnerabilities. The OpenAPI standard forbids this ambiguity to ensure that API contracts are clear, explicit, and machine-readable without guesswork.

From Linter Rules to Design Principles: Why Ambiguity Must Be Avoided

This validation error isn't just a nitpicky rule; it's enforcing a fundamental principle of robust API design. Allowing ambiguous paths violates several core tenets of building scalable and maintainable systems.

1. The Principle of Least Astonishment An API's behavior should be predictable for both humans and machines. Ambiguous paths are the opposite of predictable. An API consumer shouldn't have to wonder, "If I put a string here will it hit the username endpoint, but if I put a number it hits the user ID endpoint?" This uncertainty leads to confusion, bugs in client applications, and makes the API harder to use correctly.

2. Clear Resource Identification A core tenet of RESTful API design is that a URL should be a unique and stable identifier for a resource. Having two different path templates that could potentially resolve to the same resource introduces confusion. It muddies the clean concept of a URL as a unique address.

3. The API Tooling Ecosystem Relies on Clarity The OpenAPI Specification is far more than just documentation; it's the bedrock for a rich ecosystem of developer tools. Ambiguity breaks this entire ecosystem:

SDK Generators: How can a tool generate clear, distinct function names like getUserById() and getUserByUsername() if the underlying paths are identical? It can't, at least not without complex, non-standard extensions.
API Gateways & Routers: As we've seen, gateways are the component most directly affected. They need a deterministic contract to route traffic reliably and performantly.
Testing and Mocking Tools: When you run automated tests against a mock server, it needs to know exactly which predefined response to return for /users/some-value. If the path is ambiguous, the mock server has no way of knowing which behavior to simulate.

By forbidding identical path templates, the OAS standard forces API designers to be deliberate and explicit, resulting in a more consistent, predictable, and maintainable API surface over the long term.

Actionable Solutions: How to Resolve Duplicate Path Conflicts

Fortunately, once you understand the "why," fixing the problem is straightforward. Here are the most common and effective strategies, ranging from simple restructuring to a shift in design philosophy.

Solution 1: Add Unique Static Path Segments (Recommended)

This is the most common, explicit, and instantly understandable solution. You resolve the ambiguity by making the path structures fundamentally different by adding a static "namespace" segment.

Before (Invalid):

# openapi: 3.0.0
# ...
paths:
  /users/{userId}:
    get:
      summary: Get user by ID
      # ...
  /users/{username}:
    get:
      summary: Get user by username
      # ...

After (Valid):

# openapi: 3.0.0
# ...
paths:
  /users/id/{userId}:
    get:
      summary: Get user by ID
      # ...
  /users/name/{username}:
    get:
      summary: Get user by username
      # ...

Pro: This is absolutely unambiguous for routers and crystal clear to human readers. It's self-documenting.
Con: It makes URLs slightly longer, which is generally a negligible trade-off for the clarity gained.

Solution 2: Use Query Parameters

This solution involves a slight shift in your RESTful design philosophy. You can decide that the path parameter should only identify the primary collection (/users), and any alternative lookup method is a form of filtering that should be handled by query parameters.

Implementation:

# openapi: 3.0.0
# ...
paths:
  /users:
    get:
      summary: Get users by ID or username
      parameters:
        - in: query
          name: userId
          schema:
            type: string
          description: The ID of the user.
        - in: query
          name: username
          schema:
            type: string
          description: The username of the user.
      # ...

Your clients would then make requests like GET /users?userId=12345 or GET /users?username=jsmith.

Pro: This is a very clean and RESTful approach that completely sidesteps the path templating conflict. It's also extensible—adding another lookup method (e.g., by email) is as simple as adding a new optional query parameter.
Con: Some developers feel that using a path parameter (e.g., /users/{id}) is a stronger signal for identifying a single, unique resource than a query parameter.

Solution 3: Consolidate to a "Smart Identifier"

This approach keeps a single path template but delegates the ambiguity resolution to your backend service.

Implementation:

# openapi: 3.0.0
# ...
paths:
  /users/{identifier}:
    get:
      summary: Get user by their ID or username
      parameters:
        - in: path
          name: identifier
          required: true
          schema:
            type: string
      # ...

The backend service receiving the request for GET /users/{identifier} is then responsible for inspecting the identifier variable. It can use a regular expression or a simple check (e.g., isNumeric()) to determine if the value is an ID or a username and then call the appropriate business logic.

Pro: This keeps the API surface small and URLs clean.
Con: This pushes complexity into your backend code. More importantly, it weakens your API contract; the OpenAPI specification can no longer strongly type the identifier at the path level (though you can describe the behavior in the description).

A Warning: The Deceptive "Path Fragment" Trick

Some online guides suggest using URL fragments (#) to create uniqueness, like defining paths as /users/{id}#by-id. This is not a valid solution for routing ambiguity. URL fragments are a client-side construct processed by the browser and are never sent to the server in an HTTP request. Relying on this trick might help some documentation tools generate unique function names, but it does absolutely nothing to resolve the ambiguity for an API gateway or web server.

The API Gateway's Role: Routing, Precedence, and Abstraction

An API gateway like Apache APISIX is not just a victim of this problem; it's a key part of the solution. It is the component that must implement the routing logic with absolute precision.

High-performance gateways have a sophisticated routing engine with a clear order of precedence to resolve potential conflicts deterministically:

Exact Static Match: A request for /users/me will always be checked and matched before a templated path like /users/{id}.
Regular Expression Match: Advanced routing rules can use regex to match complex patterns.
Templated (Variable) Match: Paths with variables like /users/{id} are typically evaluated last.

graph TD
    A[Incoming Request: GET /users/me] --> G{API Gateway Router}
    subgraph Routing Precedence Logic
        G -- 1. Check Exact Match --> S["/users/me (Match Found!)"]
        G -- 2. Check Regex Match --> R["..."]
        G -- 3. Check Templated Match --> T["/users/{id}"]
    end
    S --> P1[Route to User Profile Service]

More powerfully, the API gateway can act as an "Anti-Corruption Layer." What if you are working with legacy backend services that have ambiguous paths you cannot change? This is where a gateway truly shines. It can shield your clients from this legacy complexity by exposing a clean, modern, and unambiguous API to the public, while internally routing requests to the messy services.

graph TD
    subgraph Public_API["Public API (Clean & Unambiguous)"]
        C["Client App"] --> GW("API Gateway<br/><i>Powered by Apache APISIX</i>")
    end
    subgraph Internal_Legacy_Services["Internal Legacy Services (Conflicting)"]
        S1["Service A<br/>GET /users/{id}"]
        S2["Service B<br/>GET /users/{name}"]
    end

    C -- "GET /api/v2/users/id/123" --> GW
    GW -- "Translates and Routes to" --> S1

    C -- "GET /api/v2/users/name/jsmith" --> GW
    GW -- "Translates and Routes to" --> S2

In this model, the API Gateway exposes the "correct" paths (/api/v2/users/id/{id} and /api/v2/users/name/{name}) and is configured to route them to the appropriate legacy backend. This decouples your modern public API design from your internal implementation constraints.

Conclusion: From Validation Error to Robust Design

A "duplicate path" validation error in your OpenAPI specification is more than just a minor linter complaint. It's a critical warning sign of ambiguity that violates core principles of robust API design. It creates uncertainty for your clients, breaks the automated tooling ecosystem, and leads to non-deterministic behavior at the crucial routing layer.

Fortunately, the solutions are straightforward. By using explicit static path segments, leveraging query parameters, or consolidating to a smart identifier, you can create a clear, predictable, and machine-readable API contract that benefits everyone.

Ultimately, your API gateway is a powerful ally in this endeavor. It not only enforces precise routing based on your well-defined specification but can also serve as a strategic abstraction layer. It can shield clients from the complexity of legacy systems, allowing you to present a clean API facade even when the reality behind it is messy. This architectural control is key to building and maintaining a scalable, reliable, and user-friendly API platform.